1. Field of the Invention
The present invention relates to a heat exchanger for a passive residual heat removal system. In more particular, the present invention relates to a heat exchanger for a passive residual heat removal system, capable of improving heat transfer efficiency by expanding a heat transfer area.
2. Description of the Related Art
Unlike a typical boiler, a nuclear reactor generates residual heat from the core thereof during the significant time periods after the nuclear reactor is stopped due to an accident, and the residual heat generated from the core is rapidly reduced as time elapses after the nuclear reactor is stopped.
A passive residual heat removal system has been employed for various nuclear reactors including an integral nuclear reactor, and used in order to remove the heat (the sensible heat of the nuclear reactor and the latent heat of the core) of the nuclear reactor upon the emergency situation such as an accident. In particular, a heat exchanger for the passive residual heat removal system transfers the heat received therein from the nuclear reactor to the outside (heat sink in the final step). The heat exchanger mainly employs a condensation heat exchanger based on a steam condensation phenomenon while representing the superior heat transfer efficiency.
The heat exchangers applied to most nuclear reactors employing the passive residual heat removal system are based on a water-cooled scheme as an external cooling scheme. However, some nuclear reactors employ heat exchangers based on an air-cooled scheme or both of the water-cooled scheme and the air-cooled scheme.
A water-cooled type heat exchanger represents superior cooling efficiency, so that the heat exchanger can be manufactured in small size. However, upon the accident, the water stored in an emergency cooling tank must be periodically supplemented. In addition, according to the water-cooled type condensation heat exchanger, a single phase heat transfer phenomenon is represented after steam has been condensed, so that a heat transfer coefficient is reduced. Accordingly, even if the tube length of the heat exchanger is increased, the variation of the heat transfer performance may not be greatly represented.
In contrast, according to an air-cooled type condensation heat exchanger, the emergency cooling tank is not used, and water may not be periodically supplemented. However, inferior cooling efficiency is represented, so that a large-capacity heat exchanger must be manufactured. Accordingly, the unit price of the heat exchanger is greatly increased. In addition, according to the air-cooled type condensation heat exchanger, since the heat is transferred to the outside (air) through the tube wall surface, the whole performance of the heat exchanger depends on the heat transfer efficiency of the tube wall surface (air).
Examples of the above technologies are disclosed in patent document 1 (Korean Unexamined Patent Publication No. 2010-0090990) and patent document 2 (Korean Unexamined Patent Publication No. 2009-0102079).
For example, patent document 1 discloses a passive residual heat removal system for an integral-type nuclear reactor, which communicates with a main feed water pipe and a main steam pipe connected to a steam generator so that the residual heat is removed from the integral-type nuclear reactor upon an accident. The passive residual heat removal system includes a condensation heat exchanger, which has an upper portion connected to a steam supply pipe communicating with the main steam pipe and a lower portion connected to a cooling discharge pipe, and a main valve which connects a cooling collect pipe, which communicates with the main feed water pipe, to the cooling discharge pipe and has a water receiving space formed in the operation thereof.
In addition, patent document 2 discloses a passive residual heat removal system provided with a heat pipe heat exchanger. The passive residual heat removal system includes the heat pipe exchanger connected with a decay heat exchanger through a high-temperature pipe of a sodium loop for the removal of heat to discharge the decay heat transferred thereto through the high-temperature pipe to the outside, and a sodium-air heat exchanger connected with the heat pipe heat exchanger through the high-temperature pipe to discharge the decay heat transferred thereto through the high-temperature pipe to the outside.